Decreasing cross-talk in phantom circuits



Aug. H, 1%25.

K. FISCHER DECREASING GROSS TALK I'N PHANTOM CIRCUITS Filed July 2. 1923QR W @N lfnfiantor: Kurio N Fischer, R by ,J ZZL-Z.

H i s Attorney.

Patented Aug. 11, 1925.

4 UNITED STATES 1,549,472 PATENT- DFFIE.

KUNO FISCHER, 0F CHARLOTTENBURG, GERMANY, ASSIGNOR T0 GENERAL ELECTRICCOMPANY, A CORPORATION OF NEW YORK.

DECREASING CROSS-TALK IN PHANTOM CIRCUITS.

application filed iuiy 2,

To all whom it may concern:

Be it known that I, KUNO FISCHER, a citizen of the German Realm,residing at Charlottenburg, Germany, have invented certain newand'use'ful Improvements in Decreasing Cross-Talk in Phantom Circuits,of which the following is a specification.

This invention relates to telephone transmission circuits, and moreparticularly to an arrangement of circuits whereby the socalledcross-talk effect may be reduced.

In telephone cables at the present time, the conductors ,are arranged inso-called quads, consisting of four conductors of which one pair formsone two-way transmis- 'sion circuit and the'other pair forms anothertwo-way transmission circuit, these two circuits being usuallydenominated.side

' tom. The phantom,

clrcuits. By the use of phantom resistances or choke coils, a thirdeffective circuit may be obtained from the two side circuits. This isaccomplished by arranging the circuits so that the two conductorsforming one side circuit act in parallel to form one condoctor of thethird, or so-called phantom circuit, while the other pair of conductorsconstituting the other side circuit act in parallel to form the returnpath for the phanlike the side circuits, is operated as a two-waytransmission path, that is, transmission may take place over the circuitin both directions.

It has been the practice to twist the conductors forming each sidecircuit together. to reduce cross-talk in the side circuit and also totwist the pair of conductors together to reduce cross-talk in thephantom. A similar thing is done in overhead lines by transposing pairsof conductors. While this method serves to reduce the cross-talk to someextent, it has not been entirely satisfactory. In accordance with thepresent invention, therefore, it is proposed to so arrange matters thatoutgoing and incoming conversations will take place over pairs ofconductors. belonging to different quads, at the same time taking carethat both side circuits of-agiven quad are used for transmlssion in thesame direction. This involves operation upon a four-wire basis asdlstinguished from two-wire operation, .smce four conductors arenecessary or a complete twoway transmission channel, one pair being usedfor transmission in one directlon and the same direction,

1923. Serial 648,869.

the other pair being used for transmission in another direction. Withrespect to the phantom circuit, the invention contemplates operating insuch a manner that the phantom made up from the conductors forming twoside circuits of a quad Wlll act as one side of a four-wire circuit, asimilar phantom of another quad constituting the other side of thefour-wire circuit, so that one phantom will be used fortransmission inone direction and another phantom for transmission in the reversedirection. The phantom of a particular quad should be so arranged as totransmit in the same direction as the two side circuits of the quad.

If, with matters thus arranged, the various quads which are used fortransmission in the same direction are electrically isolated from-quadstransmitting in the opposite direction, the amount of cross-talk will bevery greatly reduced. In order to understand why the cross-talk isreduced, let us consider two adjacent lines, one of which istransmitting in one direction and the other of which is transmitting inthe opposite direction. In both cases the,currents impressed upon theline at one end are greatly attenuated upon reaching the other end.Consequently the cross-talk induced in the receiv ing end of one linefrom the currents of considerable amplitude which are impressed upon theadjacent transmitting end of the other line, may have values comparablewith and even greater than the attenuated currents which are beinreceived from the transmitting end of t e same line. the other hand, ifthe two adjacent lines are operated so that transmission takes place inthe cross-talk eflect must necessarily be much smaller than the currentsactually transmitted over the line, for with respect to a given line'the current normally flowing therein w1ll be comparable in Upon Imagnitude to the current in the adjacent line which induces thecross-talk, and this holds true at any point along the line.'Consequently it follows that by grouping circuits so that both the sidecircuits and the phantoms of one group transmit in one direction whilethe side circuits and the phantoms of .the other group transmit in t eother direction, the cross-talk produced in any circuit of a group dueto induction from another circuit in the group will be of much lessconsequence than the cross-talk induced by a circuit transmitting in theother direction.

Cross-talk of the latter variety, which is known as near end cross-talk,may be practically eliminated or at any rate greatly reduced by thesegregation of the two groups of circuits. The method by which theelecrical segregation of the groups is obtained is well known in theart. For example, in a cable the conductors of one group may be shieldedfrom those in the other group in any one of a number of ways. In thecase of open-wire circuits, the two groups may be arranged upon oppositeends of the crossarms so that they are physically separated from eachother.

The invention may now .be more fully understood by reference to thefollowing description when read in connection with the accompanyingdrawing, Fig. 1 of which is a simplified circuit diagram illustratingthe principles of the invention, and Fig. 2' of which is a detailshowing how loadingmay be applied to the sideand phantom circuits ofFig. 1.

Referring to Fig. 1, twosets of quads are shown. Two conductors of onequad constitute a line LE, arranged for transmis sion from west to eastbetween stations A, and B,. Two conductors of the other quad constitutea line LW, arranged to transmit from east to west between stations B,and A,. In a similar manner the other two conductors of the first quadconstitute a line LE for transmitting from west to east between stationsA, and 13,, while the remaining pair of conductors of the other uadconstitute a line LW for transmitting from east to west between thestations 13, and A It will thus be seen that we have two completefour-wire circuits so arranged that the conductors of each quad alltransmit in the same direction.

As is usual in four-wire circuits, oneway amplifiers are included in thelines transmitting in a given direction. For ex ample, amplifiers AE,,AE, and AE, are schematically illustrated in connection with the lineLE,, and similar amplifiers are illustrated in connection with the otherlines. The. lines LE, and LW,, constituting one four-wire circuit, areconnected through the usual hybrid coil at each terminal to the terminalline leading to the stations'A and B, respectively. The ter-' minallines are balanced by artificial lines or networks N, and N, at the twotermi-,

nal stations, so that the lines LE, and LVV are substantially conjugatewith respect to each other. In a similar manner, the conductors LE andLW are connected to the terminal lines leading to the stations A; and3,. A

A phantom circuit PE is obtained from the four conductors constitutingthe lines LE, and LE of the one quad by means of drops taken from themidpoints of choke coils or resistances bridged across the side circuitsas indicated. Similar phantom connections are made at each repeatingpoint, so that a one-way phantom circuit .may be obtained in whichamplifiers PAE, PAE and PAE" are inserted as indicated. This phantomcircuit operates to transmit from station A, to station E, in adirection from west to east. A similar phantom circuit PW may beobtained from the conductors of the other quad and may be provided withone-way amplifiers for transmitting in a direction from east to Westbetween terminal stations B and A,,. The two phantom lines are connectedthrough hybrid coils to the terminal lines in a man ner similar to thatdescribed in connection with the side circuits and are renderedsubstantially conjugate by balancing the terminal lines with suitablenetworks. N and N,,. As will be noted, the phantom circuits are arrangedto transmit in the same direction as the side circuits from which they"are formed, and one complete fourwire phantom conducting systeminvolves eight individual conductors. Assuming that the quads areelectrically segregated, it will be apparent that the cross-talk'in lineLE,, for example, will be "that'induced from a line such as LEtransmitting in the same direction and will. consequently be muchsmaller in magnitude than the normal cur rents transmitted over the lineLE,.' The cross-talk induced by a line transmitting in the oppositedirection, such, for example, as the line LW. may be made negligible bythe electrical segregation of the circuits. The same holds true withrespect to the phantom. For example, the only cross-talk with which weneed be concerned in the case of the phantom PE is cross-talk induced byother phantoms transmitting in the same direction, it being assumed thatall phantom circuits transmitting in the opposite direction areelectrically segregated from those transmitting in the same direction asthe phantom PE. By this method of operation the cross-talk problem isreduced to far end cross-talk only,.the

near end cross-talk being reduced by the grouping of the circuits. Owingto its rel- ,atively small magnitude, the far end crosstalk will befully taken care of by the twistmg of the conductors in the usualmanner.

At first thought it might seem that the system herein disclosed involvesa prodigal waste-of conductors in order to obtain a reduction in thecross-talk, since it in-' volves four conductors for each fundamentalcircuit and. ei ht conductors for each phantom. Upon c oser analysis,however, it

name??? will be seen that the arranigement'is quite economical ascompared with the ordinary system in which transmission takes place intwo directions over the same circuit. In the first place, two-wireoperation involves the use of the 22 type of repeater, so that the linesmust be balanced at each repeater point, and, furthermore. since it isnot possible to efiect a perfect balance of the lines, the gain whichmay be obtained from each repeater is limited by the tendency of therepeater to sing as soon as the amplification is increased beyond acertain point. Practically, it has also been found that a limited numberof intermediate amplifiers (usually not more than 5) can be inserted intandem in the same circuit. Two-wire operation also involves aduplication of the band filters used in connection with repeaters ateach repeater point.

The four-wire circuit requires in contrast to the two-wire circuitbalancing of the line.-

at each terminal only. No balance is required at the intermediaterepeater points for the reason that one-way amplifiers may be employed.The amplifiers may be adjust-ed to obtain their maximum galn, therefore,without producing singing at the repeater point. No singing can takeplace in the four-wire circuit so long as the total attenuation in theloop formed by the two sides in the four-wire circuit is equal to orgreater than the total amplification in the loop. Consequently, with thesame number of amplifiers it is possible to use conductors of smallerdiameter, the amplifiers permitting. (because of their greater gain)agreater attenuation between rephantom when operated in manner with 22repeaters.

peater points. It has been found in practice that, by the use of thefour-wire principle, six wires of the total number of eight wires forthe phantom may be obtained Without using any more copper than would benecessary for the four wires used in the the erdinary The additionalcost of the remaining two wires of the phantomis made up in the majorityof cases by the elimination of a large number of balancing adjustmentsat the repeater points and by the elimination of the additional bandfilters, etc, required.

There remains to be considered the matter of loading the circuits. Forfour-wire operation, the side. circuits may be loaded in the same manneras in the case of twowire operation, and the usual methods may also beemployed for loading the quad. For example, the phantom pathtransmitting in one direction and comprising the four conductors of aquad, may be arran ed at the loading point as illustrated in ig. 2 inwhich windings 10 and 11 may be taken as the windings of a ringtransformer for coupling two adjacent loading sections of one ings 14and 15 of another ring transformer may be included in the drops for poseof loading the phantom.

It will be obvious that the general printhe pureiples herein disclosedmay be employed in many other organizations widely diiferent from thoseillustrated Without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. An arrangement for decreasing crosstalk in telephone lines comprisinga quad of four conductors arranged to form two side circuits and aphantom circuit, and means whereby transmission takes place over each ofthe side circuits and over the phantom circuit in one and the samedirection only.

. 2. An arrangement for decreasing crosstalk in telephone linescomprising a quad of four conductors arranged to form two side circuitsand a phantom circuit, means for transmitting in one and the samedirection only over each of the side circuits and over the phantomcircuit, and a oneway return path for each side circuit and.

.or the phantom circuit for transmission in the opposite direction,said. return paths comprising conductors of quads other than said firstmentioned quad.

3. An arrangement for decreasin the cross-talk in telephone linescomprising a quad of four conductors arranged to form two side circuitsand a phantom, a second uad of four conductors also arranged to orm twoside circuits and a phantom, means whereby both side circuit." and thephantom of said first mentioned quad will transmit in one and the samedirection only, means whereby the two side circuits and the phantom ofthe second quad transmit in the opposite direction only, and meanswhereby each of the three transmission paths formed from one quad may beassociated with a correspo ling transmission path in the other quad "isform a complete two-way transmission channel.

4. An arrangement for decreasing crosstalk in telephone lines comprisinga plurality of quads of four conductors each, each quad being arrangedto form two side circuits and a phantom circuit, means whereby the quadsmay be arranged in two groups electrically segregated from each other,means to permit transmission to take place over the side circuits andthe phantom circuits of the quads in one group in one and the samedirection only, means to permit transmission to take place over the sidecircuits and phantoms of the quads in the other group in the oppositedirection only, and means to associate each transmission path in the onegroup with a corresponding transmission path of the other group to formtwo-way transmission channels.

5. A telephone cable comprising a plurality of quads of four conductorseach, said quads being arranged to form side circuits and phantomcircuits, the various quads in the cable being arranged in two groups,means to shield the quads in one group from the quads in the othergroup, means to permit transmission in one and the same direction onlyover the. side circuits and the phantom circuits of one group, and meansto permit transmission in the opposite direction only over the sidecircuits ant phantom circuits of the other group.

In witness whereof, I have hereunto set my hand this 8th day of June,1923.

KUNO FISCHER.

